1. Field of the Invention
The present invention is related to a method of post-oxidation heating of a substrate comprising at least a SiO2 layer, or a Si/SiO2 or a Si/SiO2/poly-Si layer structure.
2. Description of the Related Art
The fabrication of insulated-gate field effect devices, like metal-oxide-semiconductor field-effect transistors (MOSFET), requires the preparation of a laterally uniform insulating silicon dioxide layer (SiO2) on the semiconductor (Si) substrate. In order to improve the electrical characteristics of this oxide layer in terms of the fixed charge density it is necessary to apply to the thermally grown oxide a post-oxidation heating cycle (see B. E. Deal, Journal of Electrochemical Society, Vol. 121, p. 198C, 1974). Next, during device structure fabrication on the semiconductor substrate, additional high-temperature processing heating steps are required. For example such heating steps are executed in order to diffuse-in or activate doping impurities in the semiconductor substrate or in a polycrystalline silicon (poly-Si) gate grown on the silicon oxide, in order to cure ion-implantation or electron-beam induced damage, in order to enhance the wafer bonding strength, and for other processing steps.
The post-oxidation heating steps are conventionally performed at temperatures above 900xc2x0 C. in an inert, non-oxidizing ambient, such as argon as suggested in document U.S. Pat. No. 3,925,107. As known in the art, such heating treatments have a negative effect on the silicon dioxide quality. These negative effects include the disintegration of thin silicon dioxide layers, development of low-voltage dielectric breakdown, and generation of hole traps in the silicon dioxide. This degradation results in an immediate impairment of the oxide as gate insulator, or hampers the MOSFET reliability through trapping of holes generated in the oxide by radiation or through trapping of holes injected from the MOSFET channel due to hot-carrier effects. Various aspects of the degradation induced by the post-oxidation heating are broadly discussed in the literature (see e.g. G. W. Rubloff et al., Physical Review Letters, Vol. 58, p. 2379, 1987; W. L. Warren et al., Applied Physics Letters, Vol. 64, p. 3452, 1994, A. Stesmans et al., Physical Review B, Vol. 54, p. R11129, 1996). The degradation is typically explained as the interface-driven formation of volatile silicon monoxide molecules (SiO) and the oxygen depletion of the silicon dioxide. In order to chemically prevent this degradation, EP-A-0264774 suggests incorporation of oxygen-containing molecules into the ambient. This method, however, is found inapplicable to thin silicon dioxide layers ( less than 10 nm-thick) as the oxygen pressure required to suppress degradation is found to be as high as 0.3 bar (cf. FIG. 4 in S. I. Raider, Microelectronic Engineering, Vol. 22, p. 29, 1993) which leads to undesirable additional oxidation.
Furthermore, the process heating steps performed after capping of the silicon dioxide layer with polycrystalline silicon or another material are further degrading the oxide quality. Various degradation aspects, including enhanced radiation sensitivity, interface defect density and 1/f noise in the Si/Silicon dioxide/poly-Si structures are known in the art (see e.g. J. R. Schwank et al., Applied Physics Letters, Vol. 53, 770, 1988; R. A. B. Devine et al., Journal of Applied Physics, Vol. 77, p. 175, 1995; V. V. Afanas""ev et al., Applied Physics Letters, Vol. 66, p. 1653, 1995). So far, the sole way to minimize the damage caused by thermal treatment of silicon dioxide layers was found to be the reduction of the temperature and duration of the thermal treatment. The incorporation of oxygen in the heating ambient is inapplicable for the buried silicon dioxides in a Si/SiO2/poly-Si structure due to the low diffusivity of the oxygen and its chemical reactivity.
In summary, the degradation of thin SiO2 layers during high-temperature processing is known in Si Metal-Oxide-Semiconductor (MOS) device technology. Upon thermal treatment (heating) at temperatures Tan greater than 600xc2x0 C. in non-oxidizing ambients, the initially superb insulator degrades in terms of characteristics such as the oxide integrity (low-voltage leaks), reduced dielectric breakdown strength, enhanced vulnerability to charging under hot-carrier injection or irradiation. These defects heavily impair the production yield of MOS devices, the device performance, and the reliability of the MOS devices. The origin of the thermal SiO2/Si degradation has been related to formation of volatile SiO molecules at the oxide/silicon interfaces. This formation is suppressed in the art by adding a small amount of an oxidant, e.g. O2, to the heating ambient. However, this method of chemical protection has a limited application as it causes additional oxidation undesirable in the case of thin gate oxides (dox less than 10 nm), because the partial pressure of O2 necessary to maintain the oxide integrity may be as high as 0.3 atm. Moreover, because of the low diffusivity of oxygen in silicon, this method cannot prevent the oxide degradation in the most widely used device structure: Si/SiO2/polycrystalline-Si.
The present invention aims to provide a method for a heating treatment which significantly reduces or eliminates, the electrical degradation of silicon oxide layers, for instance in Si/SiO2 or in Si/SiO2/poly-Si layer structures.
The present invention aims to disclose a novel physical, rather than chemical, method of oxide protection based on performing the heat treatments in an ambient of an inert gas, preferably He. The method of the invention allows to significantly reduce the degradation of both ultrathin and polycrystalline-Si covered SiO2 layers. Consequently, replacement of the currently used heating ambients with He is beneficial for the Si MOS device fabrication.
The present invention is related to a method for post-oxidation heating of at least one substrate comprising at least a SiO2 layer or a SiO2/poly-Si layer structure, comprising the steps of: creating an inert gaseous ambient in a furnace, said ambient having a partial pressure within a predetermined range and said gaseous ambient comprising molecules having a suitable diameter for penetrating into the SiO2 and/or poly-Si material; placing the substrate into said ambient; thereafter heating said furnace to a temperature of at least 200xc2x0 C. for a predetermined period of time; cooling said furnace while maintaining said gaseous ambient in said predetermined pressure range in said furnace.
The method can further comprise the step of removing said substrate from said furnace after the step of cooling down said furnace.
The heating temperature can be comprised between 500 and 1300xc2x0 C., and is preferably in-between about 550-600xc2x0 C. and about 950-1200xc2x0 C. Even more preferably the heating temperature is in-between about 750-800xc2x0 C. and about 900-950xc2x0 C.
It is an aspect of the present invention that, following the steps of the method of the invention, the degradation of both ultrathin and polycrystalline-Si covered SiO2 layers is suppressed in the temperature range of 600-800xc2x0 C. and that the degradation of both ultrathin and polycrystalline-Si covered SiO2 layers is reduced in the temperature range of 800-950xc2x0 C.
The predetermined period of time can be smaller than 1000 hours, and is preferably larger than I second and smaller than 10 minutes.
The partial pressure of said ambient is comprised between 0.05 atm and 100 atm, preferably in-between about 0.1 atm and about 5-15 atm and most preferably of about 1 atm.
The inert gaseous ambient does comprise He molecules, the He-content being larger than 99%.
In general, the efficiency of the protective action of the ambient (helium) increases with increasing ambient (helium) pressure during thermal treatment.
The substrate used in the present invention can be any suitable substrate including, but not limited to, glass, quartz, sapphire, silicon, amorphous or polycrystalline silicon, silicon carbide, polycrystalline silicon carbide, silicon nitride, aluminum nitride, gallium nitride, GaAs, AlAs, metal or metallized substrates. The preferred substrate materials are silicon, polycrystalline silicon, silicon carbide such as (0001) hexagonal silicon carbide, and polycrystalline silicon carbide. The substrates can be doped; suitable dopants of silicon carbide being, in n-type silicon carbide, nitrogen, in p-type silicon carbide, boron and aluminum.
The present invention can be executed with an overlayer present on top of the silicon dioxide layer.
The overlayers on the top of silicon dioxide can be glass, silicon, polycrystalline silicon, silicon carbide, polycrystalline silicon carbide, silicon nitride, aluminum nitride, etc. The preferred overlayer material for device applications is polycrystalline silicon.
In a best mode embodiment, the present invention is directed to device applications with MOSFET semiconductor devices, the preferred substrates are p-type or n-type silicon substrates which can have any desirable orientation, e.g., (100) or (111). The preferred dopants include (but are not limited to): in n-type siliconxe2x80x94phosphorous and arsenic; in p-type siliconxe2x80x94aluminum, boron, gallium, and indium.
The silicon dioxide layer can be provided on the substrate by any known procedure including thermal oxidation of semiconductor substrate, chemical or plasma-enhanced deposition, ion implantation or transfer by bonding from the seed wafer. The silicon dioxide thickness can be in the range of 1-1000 nm, preferably, from 1.5 to 10 nm.
After the silicon dioxide layer is provided on a substrate, it can be subjected to a postoxidation heating step, or to the deposition of an overlayer followed by a heating process step. The heating temperature can be from about 500 to about 1200xc2x0 C. Post-oxidation heating is preferable to be done in a range of about 800 to 1100xc2x0 C., most preferably from about 800 to about 1000xc2x0 C.
It is an important aspect in practicing the present invention that the inert gas ambient (He) is present in the protective ambient in sufficient concentration prior, after and during the whole thermal treatment. It is a plausible explanation of the present invention that the inert gas ambient (He) suppresses the degradation of the silicon oxide layer through physically impeding the interfacial formation of SiO. Thus the molecules of the inert gas ambient should have a diameter allowing rapid diffusion of the molecules into the silicon oxide layer and also through the overlayer, if present.
The time of thermal treatment can be chosen in the range 1 s for rapid thermal processing to 10 h for the conventional furnaces, depending on the goal of the postoxidation or processing heating. The post-oxidation heating is preferably done for 10-60 min, the processing heating for 1 min to 8 h.
The gaseous inert gas ambient (He) can be taken from any conventional source. It is most preferable to maintain the partial pressure of hydrogen-containing species in the ambient as low as possible (below 10xe2x88x926 bar) during the whole thermal treatment time including heating up and cooling down.